Re: [PATCH] mm,oom: Re-enable OOM killer using timeout.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Michal Hocko wrote:
> On Wed 20-04-16 06:55:42, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > > This patch adds a timeout for handling corner cases where a TIF_MEMDIE
> > > > thread got stuck. Since the timeout is checked at oom_unkillable_task(),
> > > > oom_scan_process_thread() will not find TIF_MEMDIE thread
> > > > (for !oom_kill_allocating_task case) and oom_badness() will return 0
> > > > (for oom_kill_allocating_task case).
> > > > 
> > > > By applying this patch, the kernel will automatically press SysRq-f if
> > > > the OOM reaper cannot reap the victim's memory, and we will never OOM
> > > > livelock forever as long as the OOM killer is called.
> > > 
> > > Which will not guarantee anything as already pointed out several times
> > > before. So I think this is not really that useful. I have said it
> > > earlier and will repeat it again. Any timeout based solution which
> > > doesn't guarantee that the system will be in a consistent state (reboot,
> > > panic or kill all existing tasks) after the specified timeout is
> > > pointless.
> > 
> > Triggering the reboot/panic is the worst action. Killing all existing tasks
> > is the next worst action. Thus, I prefer killing tasks one by one.
> 
> killing a task by task doesn't guarantee any convergence to a usable
> state. If somebody really cares about these highly unlikely lockups
> I am pretty sure he would really appreciate to have a _reliable_ and
> _guaranteed_ way out of that situation. Having a fuzzy mechanism to do
> something in a good hope of resolving that state is just unhelpful.

Killing a task by task shall eventually converge to the kernel panic.
But since we now have the OOM reaper, the possibility of needing to kill
next task is very low. Killing a task by task via timeout is an insurance
for rare situations where the OOM reaper cannot reap the OOM-killed thread's
memory due to mmap_sem being held for write. (If TIF_MEMDIE were set to all
OOM-kiled thread groups, the OOM killer can converge to the kernel panic
more quickly by ignoring the rest of OOM-killed threads sharing the same
memory, but that is a different patch.)

> 
> If I was an admin and had a machine on the other side of the globe and
> that machine just locked up due to OOM I would pretty much wanted to
> force reboot as my other means of fixing that situation would be pretty
> much close to zero otherwise.

I posted V2 of patch which also allows triggering the kernel panic via timeout.

> 
> > I'm OK with shortening the timeout like N (when waiting for the 1st victim)
> > + N/2 (the 2nd victim) + N/4 (the 3rd victim) + N/8 (the 4th victim) + ...
> > but does it worth complicating the least unlikely path?
> 
> No it is not IMHO.
>  
> > > I believe that the chances of the lockup are much less likely with the
> > > oom reaper and that we are not really urged to provide a new knob with a
> > > random semantic. If we really want to have a timeout based thing better
> > > make it behave reliably.
> > 
> > The threshold which the administrator can wait for ranges. Some may want to
> > set few seconds because of 10 seconds /dev/watchdog timeout, others may want
> > to set one minute because of not using watchdog. Thus, I think we should not
> > hard code the timeout.
> 
> I guess you missed my point here. I didn't say this should be hardcoded
> in any way. I am just saying that if we really want to do some timeout
> based decisions we should better think about the semantic and that
> should provide a reliable and deterministic means to resolve the problem.

I thought you do not like the tunable timeout because tunable timeout
leads to "what is the best duration" discussion.

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